Method for forming aluminum sheet

ABSTRACT

A PROCESS FOR THE PRODUCTION OF THIN AL-ZN-MG SHEET MATERIAL FROM RELATIVELY THICK STOCK, WHEREIN THE SHEET MATERIAL UNDERGOES A SUBSTANTIAL REDUCTION IN THICKNESS WITH NO TENDENCY FOR EDGE CRACKING OR TEARING,COMPRISING SOLUTION HEAT TREATING THE RELATIVELY THICK SHEET, COLD ROLLING THE SHEET SO AS TO PRODUCE A REDUCTION IN THICKNESS BETWEEN 40% AND 75%, PREFERABLY BETWEEN 50% AND 70%, ARTIFICIALLY AGING THE SHEET, AND COLD ROLLING THE SHEET TO THE FINAL DESIRED GAUAGE THICKNESS. ACCORDING TO ONE EMBODIMENT OF THE INVENTION, THE SOLUTION HEAT TREATMENT STEP COMPRISES CONTROLLING THE TEMPERATURES DURING HOT ROLLING TO EFFECT A DISSOLUTION OF SUBSTANTIALLY ALL OF THE ALLOYING CONSTITUTENTS AND THEN QUENCHING THE SHEET BEFORE IT IS COILED.

P 26, 1972 w. R. MOHONDRO ET 3,694,272

METHOD FOR FORMING ALUMINUM SHEET Filed Dec. 24, 1970 PERC E NT PRE-WOEKw m w 0 8 m w COLD WORK AFTER AGING EFFECT OF P06T- WORK ON TENS/LEPROPERTIES T m. P

AT VARIOUS LEVELS OF PRE-WORK L .I II3 J A K M w h 0 P 40 so cow woe/rAFTER AGING EFFECT OF P05 7- W02/( 0N TENS/LE pkopsems's .47" VARIOUSLEVELS OF FEE-WORK L IEIIE'I llEl J WILL/AM R-MOHONDRO MA UE/CE C. FE T252 INVENTOES 5v 4.04 )6;

ATTORNEY United States Patent 3,694,272 METHOD FOR FORMING ALUMINUMSHEET William R. Mohondro, Dublin, and Maurice C. Fetzer, Walnut Creek,Califi, assignors to Kaiser Aluminum & Chemical Corporation, Oakland,Calif.

Filed Dec. 24, 1970, Ser. No. 101,239 Int. Cl. C21d 7/14, 9/00 US. Cl.14812.7 13 Claims ABSTRACT OF THE DISCLOSURE A process for theproduction of thin Al-Zn-Mg sheet material from relatively thick stock,wherein the sheet material undergoes a substantial reduction inthickness with no tendency for edge cracking or tearing, comprisingsolution heat treating the relatively thick sheet, cold rolling thesheet so as to produce a reduction in thickness between 40% and 75%,preferably between 50% and 70%, artificially aging the sheet, and coldrolling the sheet to the final desired gauge thickness. According to oneembodiment of the invention, the solution heat treatment step comprisescontrolling the temperatures during hot rolling to effect a dissolutionof substantially all of the alloying constituents and then quenching thesheet before it is coiled.

BACKGROUND The present invention relates to a process for makingAl-Zn-Mg sheet from relatively thick stock without cracking or tearingduring subsequent cold rolling operations.

Prior methods for forming thin Al-Zn-M'g alloy sheet include coldrolling the hot rolled intermediate product to final gauge, solutionheat treating and subsequently aging the sheet. Due to the high workhardening rates of these alloys, it was frequently necessary to provideone or more anneals for stress relief during the cold rollingoperations. Moreover, due to the high temperatures of the solution heattreatment step, the sheet material in the final gauge thickness becomesdistorted and the surface finish is impaired which requires correctiveprocedures known to the art. The hot rolling operations usually reducethe sheet thickness to between about 0.08 and 0.20 inch. Sheet materialhaving a thickness greater than 0.20 inch is difiicult to coil and sheetmaterial thinner than 0.08 inch is usually not strong enough to be hotrolled without tearing. The final cold-rolled sheet thickness dependsupon product use but usually ranges from 0.005 to 0.06 inch.

The solution heat treating process is herein defined as heating above atemperature at which substantially all of the age hardening alloyelements are in solid solution in the aluminum matrix, i.e., above thesolvus temperature and then quenching to freeze the alloyingconstituents in solid solution. When the solution heat-treated alloy isheated to an elevated temperature, the alloying constituents precipitatefrom solid solution, thus increasing the strength and hardness of thematerial. The technique of artificial aging of heat-treatable aluminumalloys to which the present invention pertains is well known to thoseskilled in the art.

SUMMARY OF INVENTION The present invention relates to a method by whichthin sheet of an aluminum alloy containing zinc and magnesium as theprinciple alloying additives can be made without the tearing or crackingof the sheet during the cold rolling operation, and, moreover, whichwill possess sufiicient ductility in the final product to be furtherfabricated into such items as embossed roofing sheet, siding, canbodies, can ends, and the like.

3,694,272 Patented Sept. 26, 1972 The method of the present inventioncomprises solution heat treating a relatively thickaluminum-zinc-magnesium sheet material, usually between 0.08-0.20 inchthick, cold rolling the sheet so as to produce a reduction in thicknessof between 40% and preferably between 50% and 70%, artificially agingthe sheet and cold rolling the sheet to the desired gauge. Bycontrolling the degree of deformation during initial cold rolling and byartificially aging the sheet thereafter, the metal exhibits little or notendency to work harden during the final cold rolling operation. Theprocess of the present invention is directed toward an aluminum alloyconsisting essentially of 4.06.0% zinc, 0.73.0% magnesium, up to 2.5%copper, up to 0.6% manganese and balance aluminum, incidental elementsand normal impurities.

DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B are graphicalrepresentations which show the effect of the percent cold rolling afteraging on the tensile properties of the sheet at various levels ofreduction in cold rolling prior to aging.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directedtoward the process of treating an aluminum base alloy consistingessentially of 4.'06.0% zinc, 0.7-3.0% magnesium, up to 2.5 copper, upto 0.6% manganese, and the balance aluminum, incidental elements andnormal impurities. The expression incidental elements and normalimpurities, as used herein, includes iron, 0.35% maximum; silicon, 0.30%maximum; titanium, 0.10% maximum; chromium, 0.25% maximum; zirconium,0.25% maximum; other elements 0.05% each maximum, 0.15% total maximum.More specifically, the present invention is directed to the productionof sheet material having a composition consisting essentially of4.8-5.4% zinc, 0.951.3% magnesium, 0.15-0.40% manganese, less than 0.10%copper, less than 0.08% chromium, less than 0.08% zirconium, the sum ofthe chromium and zirconium not exceeding 0.10%, and the balancealuminum, incidental elements and normal impurities.

According to the present invention, after the normal pretreatments suchas homogenizing, scalping, initial breakdown and the like, the alloys ofthe present invention are hot rolled to a desired intermediate gauge,normally between about 0.08 and 0.15 inch, solution heat-treated, coldrolled so as to produce a reduction thickness between about 4075%,artificially aged and then cold rolled to the thickness desired. Thesolvus temperature for the alloys of this invention is from about 610 F.to 734 F., and thus the solution temperature must exceed 610 F. However,the temperature of the solution heat treatment should be below thetemperature at which any of the alloying phases becomes molten. For thechromiumand zirconium-free alloys, i.e., containing less than 0.10%chromium and/or zirconium, the quench rate during the solution heattreatment is not particularly critical. For example, the chromiumandzirconium-free material can be quenched using a cold or water spray,forced air and the like. Generally, however, for all the alloys of thisinvention cooling rates should be greater than 5 F. per second, at leastbetween the solvus temperature and 450 F. The artificial aging isconducted at a temperature between about F. and 350 F. for a period fromabout 6 to 72 hours. Single or multiple temperature aging practices canbe employed.

According to one particularly advantageous embodiment of the presentinvention, the temperatures in the hot rolling step are controlledso asto put into or maintain in solution the alloying constituents during hotrolling and quenching the sheet material to freeze the alloyingconstituents in solution prior to coiling of the hot-rolled sheet. Thisembodiments is particularly directed to the chromiumand zirconium-freealloy composition because these alloys are particularly insensitive toquench rate. For a successful solution heat treatment using thisapproach, it is necessary for the metal entering the hot rolling mill tobe at a temperature above the solvus temperature for the alloy, i.e.,greater than 610 F. The hot-rolled sheet generally is quenched comingoif a hot rolling mill although the quenching can occur during the hotrolling operation itself. Because this alloy is not quench sensitive, awide variety of quenching mediums can be used. For example, watersprays, soluble oil sprays, air blasts and the like. This approach isparticularly advantageous from an economic standpoint in that anadditional solution heat treat step is eliminated. However, all of thehot-rolled sheet material of this invention should be cooled below 300F., preferably below 275 F., before coiling. The material has a tendencyto age soften during storage if coiled at high temperatures. The quenchsensitive alloys can be utilized in this procedure but the quenchingrate must be quite high, i.e., greater than 275 F. per second at leastin the range between the solvus temperature and 550 LR, and the sheetmust be maintained at a temperature above the solvus temperature untilthe sheet is quenched.

The present invention uniquely provides for an economical and easilycontrolled method for producing thin sheet material which does not crackfor tear with substantial reductions in thickness during cold rolling.In accordance with this invention it has been found that by controllingthe amount of cold rolling before aging, the sheet material hassubstantially no tendency to work harden upon subsequent cold rolling.The cold rolling after solution heat treatment but before aging iscontrolled to effect a thickness reduction from about 40 to 75% By sodoing, the sheet material exhibits no substantial increase in tensile oryield strength even though the material is subjected to large additionalamounts of cold rolling, e. g., up to 70% or more.

-If the alloys of the present invention are cold rolled before aging,herein termed preworked, substantially less than 40% the sheet materialwill work harden to considerable extents during subsequent cold rolling.In addition, if the alloys are preworked more than about 75%, the sheetmaterial has a tendency to crack and tear during prework. After acontrolled amount of pework and aging the sheet material of the presentinvention can be subjected to extremely large amounts of cold rolling,herein termed postworked, with no tendency to work harden. Moreover, thepostworked sheet material can easily be fabricated into other formswithout the expensive corrective treatments required by the prior artprocesses such as straightening, surface finishing, and the like.

It has been found that a microstructure is developed during the initialcold rolling of the present invention which is characterized byintragranular deformation banding patterns. The deformation bands serveas precipitation sites during the subsequent aging treatment. The volumefraction, interparticle spacing and size of the precipitated agehardening phases of the present invention are substantially larger thanthose found when the alloy is aged with no substantial prework.Moreover, the precipitated phases are substantially noncoherent, whichare to be distinguished from the highly coherent phases which areprecipitated when the alloy is not preworked by substantial amounts. Itis believed that due to the above factors the degree of cross-slip, thematerial of the present invention can accommodate remains substantiallyconstant and therefore the work hardening rate is negligible.

The increases in tensile strength and yield strength normallyexperienced with the present invention are less than 15%. Increasedamounts of prework and smaller amounts of copper, preferably less than0.10%, provide for smaller increases in work hardening rates.

The following is an example of an embodiment of the present invention,which is intended to be illustrative and to demonstrate the inventionrather than to limit it.

The process of the present invention was applied to an aluminum alloysheet material which had been hot rolled to the intermediate gauge of0.114 inch. 'Ihesheet had the following composition:

The sheet material was solution heated to 750 F. and maintained at thattemperature for 30 minutes followed by quenching. After two days at roomtemperature, separate samples of the sheet material were cold rolled, orpreworked, 0%, 50% and 70%. After initial cold rolling, the sheet wasartificially aged using an aging practice of 195 F. for 8 hours followedby 300 F. for 8 hours. Following the aging treatment, separate samplesof the preworked sheet material which had been cold rolled 0%, 50% and70% were postworked by cold rolling 40%, 60% and 70% of the preworkedthickness. The various samples were tested for both tensile and yieldstrengths as well as for bendability. The results are given in Table I.The tensile tests vs. percent of postwork is also shown in FIGS. 1A and1B. The bend test involves bending a specimen 180 over progressivelysmaller mandrels until the specimen breaks. The results are given in thediameter of the bend in terms of the thickness of the sheet.

Before aging After aging Bend Sheet Pre- Post- Tensile Yield TensileYield test thickwork, work, str. str. str. str. thickness, percentpercent Ksd. Ks.i. Ksd. Ks.i. ness in. .3 22-; 22-:-..- a; 0 60 71I567:6 5 2 I058 70 72.4 68.9 s 5 .0348 .2 22-: a2---.-.- as 50 e0 58:056:1 310 10228 59.1 57.2 3.0 .0117 I .2 22-: as 6&9 5711 5419 310 014870 58.0 55.9 4.0 0109 From the test data it is evident that when thesheet material is subjected to the process of the present invention, thesheet material does not substantially work harden upon subsequent coldrolling even when substantial reductions in thickness occur and,furthermore, the resultant product is easily fabricated into othershapes.

What is claimed is:

1. The process of making thin Al-Zn-Mg alloy sheet from relatively thickstock comprising (a) solution heat treating relatively thick aluminumalloy sheet having a composition consisting essentially of about 4.0 to6.0% zinc, about 0.7 to 3.0% magnesium, up to 2.5% copper, up to 0.6%manganese and the balance aluminum, incidental elements and normalimpurities;

(b) cold rolling said sheet so as to produce a reduction in thicknessthereof between about 40 and 75%;

(c) artificially aging said cold-rolled sheet at a temperature betweenabout F. and 350 F. for a period between about 6 to 72 hours; and

(d) cold rolling said artificially aged sheet to the final gauge.

2. The process of claim 1 wherein the sheethas a composition consistingessentially of 4.8 to 5.4% zinc, 0.95 to 1.3% magnesium, 0.2 to 0.45%manganese, less than 0.10% copper and the balance aluminum, incidentalelements and normal impurities.

3. The process of claim 1 wherein said sheet contains less than 0.08%chromium and less than 0.08% zircomum.

4. The process of claim 1 wherein said sheet contains less than 0.10copper.

5. The process of claim 1 wherein said relatively thick sheet issolution heated at a temperature above about 610 F.

6. The process of claim 4 wherein said relatively thick sheet isquenched at a rate greater than 5 F. per second from the solvustemperature to below about 450 F.

7. The process of claim 1 wherein said cold rolling before agingproduces a reduction in thickness between about 50% and 70%.

8. The process of making thin Al-Zn-Mg alloy sheet from relatively thickstock comprising (a) preheating said aluminum alloy stock, having acomposition consisting essentially of about 4.0 to 6.0% zinc, about 0.7to 2.0% magnesium, less than 0.10% copper, up to 0.6% manganese and thebalance aluminum, incidental elements and normal impurities, to abovethe solvus temperature thereof;

(b) hot rolling said thick stock with the mill entry temperature of saidstock being above the solvus temperature for said composition;

(c) quenching said hot-rolled sheet to maintain substantially all of thealloying constituents in solid solution;

((1) cold rolling said hot-rolled sheet so as to produce a reduction inthickness thereof between about 40% and 75%;

(e) artificially aging said cold-rolled sheet at a temperature betweenabout F. and 350 F. for a period between about 6 to 72 hours; and

(f) cold rolling said artificially aged sheet to the final gauge.

9. The process of claim 8 wherein the hot-rolled sheet is coiled at atemperature below 300 F.

10. The process of claim 8 wherein the cooling rate during quenching asat least 5 F. per second from the solvus temperature to below about 450F.

11. The process of claim 8 wherein the sheet has a compositionconsisting essentially of 4.8 to 5.4% zinc, 0.95 to 1.3% magnesium, 0.2to 0.45% manganese, less than 0.10% copper and the balance aluminum,incidental elements and normal impurities.

12. The process of claim 6 wherein said sheet contains less than 0.08%chromium and less than 0.08% zircomum.

13. The process of claim 8 wherein the mill entry temperature is betweenabout 610 F. and 740 F.

References Cited UNITED STATES PATENTS 3,580,747 5/ 1971 Howitt et al14812.7 3,542,606 11/1970 Westerrnan et a1. l48--12.7 3,329,537 7/1967Loach 14811.5 X 3,346,372 10/ 1967 Jagaciak 148--11.5 X 2,506,788 5/1950 Hobbs 148--12.7

L. DEW'AYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner

